Implicit conversions occur when expressions have mixed types or function arguments do not match the function prototype. The compiler first tries a trivial conversion, then promotion, then a built-in type conversion, then a user-defined conversion, generating an error if none match. User-defined conversions are checked to see if the type needed has a defined conversion and if the supplied type matches or can be promoted. At most one built-in and one user-defined conversion will be applied. Explicit conversions occur when a cast is used, creating a temporary object that is destroyed after the statement.

1. S202 4- 1
Implicit conversions occur when mixed type
expressions are evaluated or when the actual
arguments in a function call do not match the formal
arguments of the function prototype.
First, the compiler tries to apply a trivial conversion.
If there is no match, then the compiler attempts to
apply a promotion.
If there is no match, then the compiler attempts to
apply a built-in type conversion.
If there is no match, then the compiler attempts to
apply a user defined type conversion.
If there is no match, the compiler generates an error.
Implicit Conversions

2. S202 4- 2
To determine if a user defined type conversion is used,
the compiler checks if a conversion is defined for the
type needed.
If there is, then the compiler checks that the type
matches the type supplied or that the type supplied can
be converted by applying one of the built-in type
promotions or conversions.
Only one such built-in type promotion or conversion
will be applied before applying the user defined type
conversion itself.
Thus, at most one built-in type conversion and at most
one user defined type conversion will ever be implicitly
applied when converting from one type to another.
Implicit Conversions

3. S202 4- 3
Explicit conversions occur when the client explicitly
invokes a cast operation.
Both implicit and explicit type conversions result in a
temporary object of the type being converted to.
This temporary object is used in place of the original.
The value of the original object is not affected.
When considering execution efficiency, it is important
to reduce or minimize the creation of such temporaries.
We recommend minimizing user defined conversions
as much as possible by avoiding mixed type
expressions and by supplying arguments to functions
that exactly match the type required by the function
prototypes.
Explicit Conversions

4. S202 4- 4
Constructors taking a single argument define a
conversion from the type of its argument to the type of
its class.
Such conversion functions can be used either implicitly
or explicitly.
When used implicitly, at most one implicit built-in
promotion or conversion will be applied to the
argument of the constructor. name(char* = "");
//implicitly convert char* to name
name obj; obj = "sue smith";
User Defined Type Convers.

5. S202 4- 5
Remember, the lifetime of a temporary object is from
the time it is created until the end of the statement in
which it was created.
In the assignment statement, the temporary object is
destroyed after the temporary is assigned.
As a formal argument (passed by value), the temporary
object is destroyed after the function is called.
Therefore, not only are temporary objects formed (and
the constructor implicitly invoked) but the objects are
also destroyed (and the destructors are implicitly
invoked).
User Defined Type Convers.

6. S202 4- 6
If we do not want a constructor taking a single
argument to also define an implicit conversion
function, we can prevent that by preceding the
constructor declaration with the keyword explicit.
The application is now required to provide explicit type
casts in order to convert a char* to a name object.
Using a constructor as a conversion function allows us
to convert an object of some other type to an object of a
class. They do not allow us to define a conversion from
a class to some other built-in type or class.
To do so, we must define a type conversion function.
User Defined Type Convers.

7. S202 4- 7
A conversion function allows us to define a conversion
from an object of a class to another built-in type.
Conversion functions are also useful when we need to
convert from an object of our class to an object of a class
that we do not have access to or do not want to modify.
When we do have access to the class and are willing to
modify it, we can always define a constructor taking a
single argument of our class type to perform the
conversion.
operator other_type();
User Defined Type Convers.

8. S202 4- 8
Notice that this conversion function has no return type.
That is because the return type is implicitly specified
(other_type), since that is the type we are converting to.
The conversion function converts the current object into
the new type and returns the value.
The conversion function can be called either implicitly
or explicitly.
The name of the conversion function must be a single
identifier; therefore, for types that are not a single
identifier, a typedef must first be used to create a single
identifier.
User Defined Type Convers.

9. S202 4- 9
Notice that this conversion function has no return type.
That is because the return type is implicitly specified
(other_type), since that is the type we are converting to.
The conversion function converts the current object into
the new type and returns the value.
The name of the conversion function must be a single
identifier; therefore, for types that are not a single
identifier, a typedef must first be used. In the class:
typedef const char* pchar; //make single identifier
operator pchar(); //conversion (name to char*)
Implementing:
name::operator pchar() { //conversion function
...
return array; //of type char * being returned
}
User Defined Type Convers.

10. S202 4- 10
In order for the conversion function to be called
explicitly when it is a typedef name, that name must be
in the global namespace and cannot be hidden within
the class.
name obj("sue smith");
const char* p;
//Three examples of explicit conversions:
p = (pchar)obj;
p = pchar(obj);
p = static_cast<pchar>(obj);
User Defined Type Convers.

11. S202 4- 11
With constructors as conversion functions and also
conversion functions, avoid mutual conversions.
Many times type conversions are specified to minimize
the number of operators overloaded, especially for
symmetric operators where there can be multiple
combinations of types for the two operands. It can
reduce the number of versions of an operator to a
single, non-member.
However, this can produce subtle changes in the way a
program works. If we were to add a string to char *
user defined conversion function to our string class, all
uses of operator+ containing both char * and string
types would become ambiguous.
User Defined Type Convers.

12. S202 4- 12
Type conversions will not help when operators are
members -- because a member is required to have the
first operand as an object of the class; if it converted to
some other type, the member function will not be
found as a possible candidate function and a compile
error will result or the wrong operator will be used
(such as the built-in operators).
Also, when user defined conversions to built-in types
have been specified, the predefined operators may end
up being used even when we use an operator with a
user defined type for which an overloaded operator
exists! Overloaded operators are only invoked if at least
one of those operands is a user defined type.
User Defined Type Convers.